This invention relates to a composition for use in a rearview mirror of the type containing a fluid light controlling medium. More particularly, the invention relates to a fluid composition for use in a vehicle rearview mirror of the day-night type, which utilizes an optically dense light attenuating fluid in the night position of the mirror capable of inhibiting glare and which will diminish or block out reflections of the mirror so as to avoid double images hazardous to driving at night.
Three components are common to known fluid controlling media for use in rearview mirrors, namely (a) a carrier fluid, (b) a light absorbing medium, and (c) optionally a stabilizing medium. The three components can conceptually form several types of fluid and more particularly an emulsion, a solution or suspension. Of the possible fluid types it would be expected that a solution would be the most stable over long periods of time viz periods of time in the region of ten years.
Rearview mirrors containing a fluid light controlling medium are known, for example, from U.S. Pat. Nos. 3,198,070 and 3,233,515. U.S. Pat. No. 3,198,070 recommends the use of a colloidal suspension of fine carbon black particles dispersed with the aid of rubber molecules in kerosene or comparable solvent. However, the colloidal suspension is not U.V. stable and furthermore staining of the glass and mirror components of a rearview mirror containing such a suspension is observed. In U.S. Pat. No. 3,233,515 the fluid medium is described as "an opaque optical fluid" such as that described in concurrently filed U.S. Application Ser. No. 156,535.
To date, we are unaware of any rearview mirror containing a fluid light controlling medium in the form of a solution.
Studies carried out by us have indicated that factors pertinent to the selection of the carrier fluid for a rearview mirror of the day-night type, which utilizes an optically dense light attenuating fluid, include (not in order of priority) viscosity, vapor pressure, the temperature coefficient of viscosity and vapor pressure, boiling and melting points, flash point, surface tension, U.V. stability, solubility characteristics, chemical inertness, cost toxicity, refractive index and aeration. U.S. Pat. No. 3,198,070 recognizes that preferably the fluid should be stable over a wide range of temperatures viz -30.degree. F. to 200.degree. F. (-23.degree. C. to 93.degree. C.). However, as indicated above, in addition to temperature stability, other very pertinent factors include viscosity, U.V. stability, chemical inertness and toxicity.
It is an object of the present invention to provide an anti-glare, day-night mirror which incorporates a stable, optically dense fluid which prevents transmission of light to and from the surface of the mirror at night an thus reduces glare to a minimum.
Accordingly, the invention provides a fluid composition for use in a rearview mirror of the type containing a U.V. stable fluid light controlling medium, said composition comprising:
(a) a carrier fluid consisting of a solvent or solvent mixture having a freezing point .ltoreq.-30.degree., a boiling point .gtoreq.80.degree. C., and a viscosity within 5.degree. C. of the freezing point thereof of up to 50 centipoise; PA1 (b) a light absorbing medium soluble in the carrier fluid and consisting of a dye or mixture of dyes, said dye or mixture of dyes remaining in solution at a temperature below -30.degree. C. and having an absorbance characteristic when combined with the other fluid components according to Beer-Lambert's Law: EQU Ix=Ioe.sup.-.alpha.cx PA1 Ix=intensity of illumination after traveling through the fluid; PA1 Io=initial intensity of illumination; PA1 .alpha.=average absorbance coefficient of the dye or dye mixture in the visible range of radiation; PA1 c=concentration of the dye or dye mixture; PA1 and x=light pathlength through the light absorbing medium such that EQU .alpha.cx=A
wherein
where A has a value in the range 0.05 to 11.0 and where the maximum value of A for the day position of the mirror is 0.28 and where the minimum value of A for the night position of the mirror is 2.00; and
where x.sub.1 is the in use fluid thickness and is defined by the equation EQU x.sub.1 =L Cos .theta.
in which 2L is the pathlength of the light through the fluid and .theta. is the angle at which the incident light strikes the mirror; and x =2L; and (c) optionally a U.V. stabilizing agent.
Preferably, the freezing point of the solvent or solvent mixture is .ltoreq.40.degree. C. and the boiling point of the solvent or solvent mixture is .gtoreq.120.degree. C.
A preferred solvent is propylene carbonate and a preferred solvent mixture is propylene carbonate/ethanol.
Apart from the essential features of the carrier fluid specified hereinabove, the fluid should be nontoxic, easy to handle and have a relatively high flash point. Furthermore, the fluid should be relatively inert to glass, a range of rubbers, and plastics and other engineering materials, such as steel, used in the construction of a rearview mirror. The carrier fluid should preferably be relatively U.V. stable and a low vapor pressure is also advantageous. A further preferred characteristic of the carrier fluid is a refractive index which is similar to that of commercial glass. The carrier fluid must also obviously have the ability to dissolve the dye o dye mixture and the U.V. stabilizing agent, if present.
Preferred dyes include azo, anthraquinoid, phthalocyanine and 1,2-chrome complex dyes and mixtures thereof
The dye must remain in solution over the temperature range specified for the solvent or solvent mixture. The dye must have a long period of light fastness and be capable of remaining in solution over several years without fading. Specifically, the dye must be completely soluble in the solvent or solvent mixture and should be capable of remaining in solution in the solvent or solvent mixture for a period of approximately ten years under extreme conditions. The dye or dye mixture is preferably relatively U.V. stable.
The dye or dye mixture should be fairly uniform and optically dense in the visible region of the spectrum, with greatest density in the region with the highest product of eye sensitivity and automobile headlight intensity.
In particular, a preferred dye mixture has been found to be a mixture of Lampronol Black BR and Savinyl Blue GLS in a ratio of 0.6:1 by weight as hereinafter described in the Example. Lampronol and Savinyl are trademarks.
In addition to the light absorbing medium remaining in solution in the solvent or solvent mixture over a period of, for example, ten years, it must also remain optically intact, which may involve the use of a U.V. stabilizing agent.
Examples of suitable U.V. stabilizing agents are benzophenones, benzotriazoles or organo-nickel complexes. A preferred stabilizing agent is Cyasorb U.V. 5411 (Cyasorb is a trademark of the American Cyanamid Company). The primary job of the stabilizing agent is to give U.V. stability. Several U.V. stabilizing agents may be used so that all possible stabilizing mechanisms are exploited to the full, for example, absorption and scavenging.
The products of U.V. degradation of the dye or carrier fluid should be nonaggressive to components of the fluid or the mirror and should remain soluble.
Other constraints to those mentioned above in selecting a suitable dye for use in the composition according to the invention, include the fact that the dye should not stain the glass sealing plate or the mirror or mirror glass. Furthermore, the constituent particles of the dye must not agglomerate. Other factors which must be considered are the aesthetic appearance of the mirror and cost factors.
A solution of the dye or dye mixture in the solvent or solvent mixture should give an optically dense fluid over the visible spectrum using a short pathlength (circa 2 to 4 mm).
The composition according to the invention is suitable for use in interior or exterior vehicle rearview mirrors of the type referred to. Specifically, the composition according to the invention is used as the optically dense light attenuating fluid which is used in the night position of a day-night rearview mirror, to provide a layer of nonreflecting opaque fluid between the mirror and a clear glass sealing plate. The composition according to the invention can be used with any such day-night rearview mirror, but especially with the day-night rearview mirrors of the type described in U.S. Pat. Nos. 3,198,070 and 3,233,515. The composition according to the invention is particularly suited for use with the rearview mirror of our co-pending European Pat. Application No. 85309063.7 (0186391).
As stated above, propylene carbonate is especially suitable for use as the carrier fluid in a day-night mirror of the type referred to. A list of the physical properties of propylene carbonate illustrating its suitability are listed in Table 1.
The temperature dependence of vapor pressure for propylene carbonate is such (see Table 1) that the boiling/melting range is exceptionally wide; 242.degree. C. to -49.degree. C. The temperature dependence of viscosity (see Table 1) is modest. Furthermore, propylene carbonate has a room temperature vapor pressure of 0.03 mm Hg at 20.degree. C. which is particularly low. Furthermore, with a polarity index of 6.2 (compared with, for example, water at 10.2 and pentane at 0.0), propylene carbonate is a good solvent for many dye systems. Furthermore, U.V. stabilizers such as benzophenones, benzotriazoles and organo-nickel complexes are also all readily soluble in propylene carbonate.
Since vapor pressure (and its temperature dependence) is exceptionally low for propylene carbonate, the potential for vapor buildup is reduced. The viscosity of propylene carbonate (and its dependence on temperature) is low, hence fluid movement and thus mirror response is rapid across a wide temperature range.
The refractive index of 1.421 at 20.degree. C. is close to that of soda lime glass (1.52) and hence helps to reduce secondary reflections. Propylene carbonate shows a sharp U.V. cut-off at 280 nm which helps to shield dissolved dyes. The cubic coefficient of expansion of 0.00096 cc/cc/.degree. C. is modest. Propylene carbonate cools easily and remains liquid at temperatures as low as -80.degree. C. for a period of several months.
Other properties of propylene carbonate relevant to its use as a solvent in the compositions of the present invention are that it is a relatively inert solvent with a pH of 6.4. It is considered practically nontoxic (L.D. 29 g/Kg when administered orally to rats). It is almost odorless. Splashes of propylene carbonate to the eyes can be painful. It is not a skin poison but can be painful if it enters an open wound, similar to a disinfectant. There are no special requirements for storage or handling. The flash point is high for an organic solvent (132.degree. C.) compared to -17.degree. C. for petroleum spirits. It has an auto-ignition temperature of 510.degree. C. Fires can be extinguished with water, carbon dioxide, dry powder, foam or vapor liquids.
Accordingly, it will be appreciated that propylene carbonate is particularly suited for use as a solvent in the composition according to the present invention.
At low temperature (-30.degree. C.) the viscosity of propylene carbonate is approximately 20 centipoise. Accordingly, in order to ensure an improved viscosity, addition of a solvent such as methanol, ethanol, isopropanol, 1-butanol, 2-pentanol, n-amyl alcohol, isoamyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone, gamma butyrolactone, anisole, p-cymene, dimethyl formamide and amyl acetate may be used. For example, a significantly lower viscosity will be obtained by using propylene carbonate and ethanol in an amount of 80:20 v/v.
The invention will be further illustrated by the following Example.